The present invention relates generally to an oxygen sensor for detecting the oxygen concentration of a measurement gas, especially an exhaust gas produced by an internal combustion engine. More particularly, the invention is concerned with such an oxygen sensor which has an elongate sensing element having an oxygen detecting portion at its one longitudinal end thereof and electrical terminal means at the other longitudinal end thereof, wherein the sensing element is accommodated in a tubular protective means such that the oxygen detecting portion is exposed to a measurement gas.
There has been known an oxygen sensor which detects or determines the oxygen concentration of an exhaust gas emitted from internal combustion engines, for the purpose of controlling the combustion of fuel burning conditions of the engine according to signals produced by the oxygen sensor, and thereby purifying the exhaust gas and minimizing fuel consumption of the engine. An example of such oxygen sensors uses a sensing element which comprises a body of oxygen-ion conductive solid electrolyte such as zirconium oxide doped with calcium oxide or yttrium oxide, and further comprises suitable electrodes disposed on opposite surfaces of the solid electrolyte body. In this oxygen sensor, one of the electrodes is exposed to a reference gas while the other electrode is exposed to the exhaust gas. In operation, the oxygen sensor produces an output signal which represents an electromotive force induced between the two electrodes according to the principle of an oxygen concentration cell. In recent years, there has been an increasing tendency to use an elongate planar sensing element rather than a conventionally used tubular sensing element, in view of ease of manufacture and structural simplicity of the sensor. Such an elongate planar sensing element has, at its one end, an oxygen detecting portion to be exposed to an exhaust gas or some other type of measurement gas. Such planar oxygen sensing elements and oxygen sensors incorporating a planar oxygen sensing element are disclosed, for example, in U.S. Pat. No. 4,450,065, and Japanese Patent Application laid open in 1983, under Publication No. 58-172542.
Oxygen sensors of various types as discussed above are installed such that the oxygen detecting portion of their oxygen sensing element is generally exposed to an exhaust gas or other measurement gas of an elevated temperature. Further, the sensing element is positively heated by a built-in electrical heater so as to maintain its detecting portion at a high temperature so that the detecting portion may function to effect accurate oxygen detection of the measurement gas. Thus, the entire portion of the sensing element is exposed to a relatively high temperature. Additionally, electrical leads for an electrical output indicative of the oxygen partial pressure of the measurement gas, and electrical leads for power supply to the electrical heater, are electrically connected to external conductors which extend from the oxygen sensor for electrical connection with external devices. The above-indicated exposure to heat of the sensing element results in difficulties and inconveniences in making electrical connections between the electrical leads of the sensing element and the external conductors.
For instance, the leads of the sensing element are electrically connected to connectors of a spring material such that the leads and the connectors are held in resiliently pressed contact with each other. These electrical connectors are electrically connected to suitable external conductor members. Such electrical connections by means of pressed contact of the leads with the connectors suffer a fatigue of the resilient connectors and oxidation of the contacts due to exposure to heat. An alternative method of electrical connection is available, wherein the external conductor members are brazed to terminals of the leads. This method, however, has problems that a brazing material such as Ag undergoes migration, and the brazed portion are likely to crack or flake off. In either case, the oxygen sensor of the type indicated above is not completely satisfactory in reliability of its electrical connections.